What lies immediately outside of the heliosphere in the very local interstellar medium (VLISM): What will ISP detect?

The Interstellar Probe (ISP) will provide the first direct
measurements of interstellar gas and dust when it travels far beyond the
heliopause where the solar wind no longer influences the ambient medium.
We summarize in this presentation what we have been learning about the VLISM
from 20 years of remote observations with the high-resolution spectrographs
on the Hubble Space Telescope. Radial velocity measurements of interstellar
absorption lines seen in the lines of sight to nearby stars allow us to
measure the kinematics of gas flows in the VLISM. We find that the heliosphere
is passing through a cluster of warm partially ionized interstellar clouds.
The heliosphere is now at the edge of the Local Interstellar Cloud (LIC) and
heading in the direction of the slighly cooler G cloud. Two other warm clouds
(Blue and Aql) are very close to the heliosphere. We find that there is a
large region of the sky with very low neutral hydrogen column density, which
we call the hydrogen hole. In the direction of the hydrogen hole is the
brightest photoionizing source, the star Epsilon Canis Majoris (CMa). Extreme
ultraviolet photons from this star produce a Stromgren sphere region of
ionized gas as large as the Local Cavity (extending to 100-200 parsecs)
and produce Stromgren shells at the outer regions of the local warm clouds
including the LIC.

When the ISP passes beyond the hydrogen wall at a distance of about 500 AU,
it will likely enter the outer edge of the LIC where photoionization from
Epsilon CMa plays an important role. Analysis of Hubble observations of
interstellar absorption proves estimates of the densities, temperature,
pressure, and flow properties of the main portion of the LIC, but we have
little informtion on these properties at the LIC's edge. Comparison with the
inflow vector of neutral helium measured by IBEX and Ulysses indicates a
slightly different flow speed and direction than the mean flow of the LIC gas.
ISP will provide direct measurements of the flow and gas properties of this
poorly understood region. In particular, ISP will provide information on
how photoionization from Epsilon CMa influences warm clouds through ionization,
heating, and perhaps pressure balance. This information may resolve questions
concerning the magnetic field surrounding the heliosphere.